The present invention relates generally to wireless communication systems and more particularly to techniques for performing inter-frequency handoffs in wireless code division multiple access (CDMA) systems and other types of wireless systems.
An important issue in IS-95 CDMA cellular and Personal Communications Service (PCS) wireless communication systems involves frequency handoffs between adjacent cells or antenna sectors. Each of the cells in such a system generally includes a base station, and the base station associated with a given cell may include an omnidirectional antenna or a multiple-sector directional antenna for communicating with mobile stations such as portable telephone handsets. As a mobile station moves throughout the system, its position relative to the system base stations changes, such that an ongoing call or other communication may need to be handed off from one base station to another, or from one antenna sector to another. Adjacent base stations and antenna sectors are typically configured to utilize different communication frequencies in order to minimize effects such as co-channel interference. Handoffs from one cell or sector to another may therefore involve changing the communication channel frequency from a current frequency to a new frequency. Such handoffs are generally referred to as inter-frequency or other-frequency handoffs.
A number of techniques have been proposed for improving the efficiency and success rate of inter-frequency handoffs in IS-95 CDMA systems. These techniques utilize messages such as Extended Handoff Direction Messages (EHDMs), Other Frequency Neighbor List Messages (OFNLMs), Other Frequency Neighbor List Response Messages (OFNLRMs) and Other Frequency Report Messages (OFRMs) to implement inter-frequency handoffs. Although proposed techniques based on these messages can reduce the number of call drops during a frequency transition, these techniques still suffer from a number of drawbacks. For example, the OFRM message is presently configured to allow a mobile station to report a signal-to-noise measure which is both xe2x80x9cinterference limitedxe2x80x9d in that it will typically decrease as the mobile moves across same-frequency cell boundaries, and xe2x80x9cnoise limitedxe2x80x9d in that it will also decrease as the mobile moves across other-frequency cell boundaries. For a measure which is interference limited, interference due to signals generated by other cells is greater than the noise level, while for a measure which is noise limited, the noise level is greater than the interference due to signals generated by other cells. A same-frequency cell boundary may be defined by a set of points at which the strength of a pilot signal from one cell exceeds that of a pilot signal from an adjacent cell, where both the pilot signals are at the same frequency. An other-frequency cell boundary may be defined as a set of points at which a signal from one cell at a designated frequency exceeds the strength of a signal from an adjacent cell at another frequency by a specified number of decibels. An OFRM message which utilizes conventional signal-to-noise measures cannot be used to distinguish same-frequency cell boundaries from other-frequency cell boundaries, and therefore does not provide an optimal trigger for inter-frequency handoffs.
In addition, the above-noted message-based techniques will often involve a base station commanding a mobile station to perform a periodic search for a new frequency as soon as the mobile station enters into a transition area near the edge of a new cell or sector. However, this periodic search tends to degrade voice quality of an ongoing call, while also reducing the speed of the search for new potential base stations at the current frequency. Moreover, in many practical applications, this periodic search for a new frequency can be unnecessary if the mobile is operating under certain types of radio frequency (RF) conditions. Yet another significant problem with the above-noted techniques is that the techniques may increase the likelihood of xe2x80x9cping-pongingxe2x80x9d or rapid switching between the new frequency and the current frequency. More particularly, it may be possible in some areas of the system that both the new frequency and the current frequency will have good RF coverage, which could lead to ping-ponging if, for example, the mobile station reports the received power and signal-to-noise measure for only the new frequency.
The invention provides methods and apparatus for improving inter-frequency handoffs in CDMA and other types of wireless communication systems. In accordance with a first aspect of the invention, a noise-limited coverage trigger is provided which may be used to distinguish between same-frequency cell boundaries, which are generally interference limited, and other-frequency cell boundaries, which are generally noise limited. The coverage trigger is used to control inter-frequency handoffs, and can be implemented using signal-to-noise measurements performed in a mobile station. In an illustrative embodiment, the coverage trigger may be generated as the difference between the average transmit signal-to-noise measure for all significant pilots and the linear sum of significant pilot signal-to-noise measures reported in a Power Measurement Report Message (PMRM) or Pilot Strength Measurement Message (PSMM) transmitted from the mobile station.
Alternative embodiments may utilize the mobile receive power alone as a trigger metric. For example, a measure of mobile receive power incorporated into a PSMM can be used to trigger a handoff to another frequency using a xe2x80x9cdatabasexe2x80x9d approach. In this approach, when the mobile receive power in a given cell becomes small and the mobile sees primarily border cell pilots, a particular pilot is selected from a list of neighbor pilots stored in a database for that cell, and the mobile is instructed to perform a xe2x80x9cblindxe2x80x9d handoff to the selected pilot at the new frequency. In this manner, a mobile can be instructed to perform a handoff to a new frequency without taking any pilot Ec/Io measurements at the new frequency. The mobile receive power can also be used in the mobile to filter periodic reports. For example, the mobile may only make PSMM reports when the mobile receive power drops below a threshold, which may be specified by the current cell site.
Other aspects of the invention provide alternative techniques for controlling inter-frequency handoffs in a wireless communication system. These techniques can substantially eliminate unnecessary periodic searches in a frequency transition area, while also reducing the likelihood of ping-ponging between a current frequency and a new frequency. In an illustrative embodiment, the invention involves adding additional field parameters to an Extended Handoff Direction Message (EHDM) of an IS-95 CDMA system. The additional field parameters include one or mare thresholds which ensure that a mobile station initiates a search for a new frequency, for example, only if its received power at the current frequency is lower than a certain threshold, or if a sum of signal-to-noise values for its active pilot signals at the current frequency is less than a certain threshold. The mobile can also be configured to operate such that if the received power at the new frequency does not exceed the received power at the current frequency by a designated hysteresis amount, then a search is not performed in the new frequency. The invention thus provides additional checking criteria for use in inter-frequency handoffs to ensure that unnecessary tuning and search in the new frequency are avoided.
Further improvements can be provided in inter-frequency handoffs in other illustrative embodiments of the invention. For example, a mobile station in an area in which a transition is to be made from a current frequency to a new frequency is configured to report received power and signal-to-noise values for both the new frequency and the current frequency. If these values indicate that the mobile station is operating under acceptable RF conditions at the current frequency, there is no need to perform an inter-frequency handoff. This aspect of the invention can substantially reduce the likelihood of ping-ponging between the current and new frequencies, as compared to the conventional inter-frequency handoff techniques noted above.